Metallic materials capable of being deformed and then of recovering their original shapes when heated are well established and are known as shape memory alloys. These alloys exhibit a reversible crystalline phase transformation from a high temperature phase of austenite to a low temperature phase of martensite. The temperature at which the transformation begins is referred to as M.sub.s while the finishing temperature of the transformation is referred to as M.sub.f. The starting and finishing temperatures associated with the reverse transformation from martensite back to austenite on heating are referred to as A.sub.s and A.sub.f, respectively. When these alloys are deformed at a temperature not far above the A.sub.f temperature, yielding occurs through the formation of stress induced martensite. Since martensite is mechanically unstable at these temperatures, it reverts back to austenite upon the release of deformation. As a result, it is possible to achieve elastic recovery of strains as great as 8%; this is referred to as "pseudoelasticity". Stress-induced martensite, and hence the deformation, can be stabilized by cooling. On the other hand, when shape memory alloys are deformed in their martensitic phase, the deformation proceeds by texturing the martensite crystalline along an orientation that is mechanically favorable to the deformation. In both cases, shape memory alloys retain the shape of deformed martensitic structure until subsequent reverse transformation on heating during which the alloy recovers its original shape. This thermally-induced shape recovery is termed "shape memory effect".
Both pseudoelasticity and shape memory effect have been observed in several alloy systems. Some of those considered to have commercial merits are NiTi, CuZnAl, CuAlNi, and their ternary, quaternary and higher order derivatives. These shape memory alloys, in particular NiTi based alloys, have found many commercial applications in, for example, thermal and electrical actuators, fluid pipe couplings, electrical connectors, orthodontic arch wires, and many other medical devices. These applications are reviewed in Shape Memory Alloy Applications by L. McDonald Schetky, in Intermetallic Compounds (chapter 26), Vol. 2, Practices, pp26.1-26.30, 1994, Edited by J. H. Westbrook and R. L. Fleischer, John Wiley & Sons, Ltd.
Depicted in FIG. 1 are two stress-strain curves of a shape memory alloy. Curve (1), typical in the vicinity of its phase transformation temperature, exhibits two stages of yielding with the first one related to stress-induced martensite transformation or texturing of existing martensite. Curve (2), typical at temperatures higher than M.sub.d, the maximum temperature at which the martensite can be stress-induced, exhibits a conventional one stage yielding behavior. As in FIG. 2, the first yield stress (.delta..sub.y) when plotted against temperature reaches a maximum at the M.sub.d temperature. Softening on cooling is attributed to the stress-induced transformation of martensite with (.delta..sub.y) approaching a minimum at the M.sub.s, temperature. Decreasing yield stress on heating above the M.sub.d temperature, on the other hand, is related to material softening at higher temperatures.
In applications such as fluid pipe couplings and various connectors, two mechanical elements are jointed together by the shape recovery of a shape memory alloy article. The strength of such a joint relies on the shape memory article to maintain its mechanical strength over the entire range of service temperature. As illustrated in FIG. 2, the service temperature of these applications is defined by the range where the yield strength of shape memory alloy exceeds a minimum requirement .delta..sub.m, for maintaining proper mechanical integrity in these applications. This range of service temperature is bounded by a lower limit of T.sub.l and an upper boundary of T.sub.u in FIG. 2.
For military fluid couplings where the specification demands that the coupling be functional down to -55.degree. C., a cryogenic alloy such as a NiTiFe alloy with M.sub.s temperature well below -55.degree. C. is used (U.S. Pat. No. 4,035,007). The cryogenic NiTiFe alloy coupling is expanded at liquid nitrogen temperature (-196.degree. C.). To prevent premature shape recovery, the deformed coupling must be stored and transported at a cryogenic temperature before installation. For convenience, it is preferable to use a shape memory alloy with its M.sub.s, temperature at, for example, below -100.degree. C., and its A.sub.s temperature above the maximum possible ambient temperature of storage and transportation, for example, 50.degree. C.; i e., a wide transformation hysteresis of 150.degree. C. Articles made of such an alloy can be processed, stored and transported at ambient temperature in the martensitic condition without the risk of premature shape recovery. During installation, heating would be applied to induce shape recovery; the process is referred to as "heat-to-recover".
Both CuZnAl and CuAlNi alloys have a hysteresis about 15-20.degree. C. Near equiatomic binary NiTi alloys have a hysteresis about 30-40.degree. C. The width of the hysteresis of NiTi based alloys can be manipulated by alloying. Adding copper to binary nickel-titanium alloy reduces the hysteresis to 10-20 .degree. C. (such as described in Cu-Content Dependence of Shape Memory Characteristics Ni--Ti--Cu alloys, by Nam et al, in Materials Transactions, Japan Institute of Metals, vol.31, No.11, pp. 959-967, 1990) while adding iron to the binary alloy widens the hysteresis to approximately 70.degree. C. (as described in U.S. Pat. 3,753,700). However, a hysteresis of 70.degree. C. is not sufficiently wide to realize the convenience of the heat-to-recover process. These alloys are still limited by cryogenic storage and installation in applications at subambient temperatures. It was recently disclosed in U.S. Pat. No. 4,770,725 that by adding niobium to binary nickel-titanium alloys, it is possible to obtain a cryogenic M.sub.s temperature together with a widened hysteresis. It was disclosed that in some copper based shape memory alloys, the hysteresis can be temporarily expanded by a "preconditioning" process consisting of mechanical and thermal processing (U.S. Pat. No. 4,095,999). It was also published that overdeforming a binary nickel-titanium alloy can temporarily shift both A.sub.s and A.sub.f to higher A.sub.s ' and A.sub.f ' temperatures (Miyazaki et al., Transformation Pseudoelasticity and Deformation Behavior in Ti-50.6 at. %Ni Alloy, Scripta Metallurgica, vol. 15, no. 3, pp. 287-292 (1981)). It was further disclosed in U.S. Pat. No. 4,631,094 that by mechanically preconditioning a nickel-titanium-niobium alloy, the reverse transformation temperatures after preconditioning, A.sub.s ' and A.sub.f ', can be shifted to a range above the ambient temperature, thereby making practical the heat-to-recover installation process. Preconditioned nickel-titanium-niobium alloys with widened hysteresis are used in heat-to-recover couplings and connectors (L.McD. Schetky, The Applications of Constrained Recovery Shape Memory Devices for Connectors, Sealing and Clamping, Proceedings of the First International Conference on Shape Memory and Superelastic Technologies, Pacific Grove, Calif. (1994)). These devices carry a common trade name of UniLok.RTM.. Heat-to-shrink UniLok.RTM. connector rings are also used for clamping a thin-walled can onto a base for packaging various electronic devices and at the same time providing a hermetic seal. The ring is especially attractive for attaching thin walled metal cylinders to bases of different materials, such as ceramics, plastics and dissimilar metals.
High pressure fluid passages, such as those in diesel fuel injectors, may experience operating pressures as high as 30,000 psi. To achieve even better efficiency of fuel combustion, next generation injectors may see even higher operating pressure than 30,000 psi. In order to machine internal high pressure fuel passages a hole must be bored in the outer injector body and after completion of the machining this hole must be sealed. The conventional method for sealing the opening after the machining operation is by brazing a steel plug into the hole. However, brazed plugs often have defects which are difficult to detect and may cause burst or fatigue failure in testing or in service. A more reliable sealing method which can withstand higher pressures and exhibits a longer fatigue life is therefore highly desirable. The present invention discloses an application of shape memory alloys for sealing a fluid or gas passage or opening capable of withstanding a wide range of operating pressures with a longer fatigue life.